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Modeling the Seismic Noise Horizontal-to-Vertical Spectral Ratio in Laterally Irregular Configurations Using the Diffuse Field Assumption

Description: 

The horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise (ASN) is a current measure to assess the predominant soil frequency of a given site. We assume that ASN field comes from equipartioned, uniform illumination of random sources and the scattering by heterogeneities leads to some equilibration that makes a field diffuse. Besides, diffuse fields are closely related with the Green’s function. Under the diffuse field assumption (DFA) theory asserts that the average directional energy densities (DED) are related to Green's function. Therefore, using ASN and under the DFA, the HVSR is computed with the imaginary parts of the Green’s function. That fact made the inversion of soil properties in depth feasible for horizontally layered media. However, this is not completely free of uncertainties about the diffuse character of the wavefield and the non-uniqueness of the inversion, subjects that require further scrutiny. In practice, lateral variations are ignored or integrated with some interpolation. Their study may add physical meaning to the quest because HVSR in irregular layered settings has significant lateral and azimuthal variability. In this work, we model HVSR under the DFA and use the 3D indirect boundary element method (IBEM) to study an irregular layer and use an adaptive mesh scheme by accounting for the locality of problem and the wave diffraction properties. Our preliminary results show that layer irregularities may lead to “spots” of enhanced wave excitation and suggest the need for denser field measurements to study lateral variations and azimuthal variability. Clearly, the potential variations of possible substructures greatly exceed any information present in HVSR. As ever, one must supplement with extra information, possibly non-seismic, and learn to live with some uncertainty. We discuss results and examine the implications for seismic hazard assessment.

Session: Numerical Modeling in Seismology: Developments and Applications - I

Type: Oral

Date: 5/1/2024

Presentation Time: 08:45 AM (local time)

Presenting Author: Francisco

Student Presenter: No

Invited Presentation: 

Authors